Wife and I recently got a Trel T-900 tandem. She has an OSI spare tire rack which clamps to the top tube of the bike. We center it on the balance point. I havean old Saris rack which has the "bat wings" Saris sent me a set of new ones after the original ones failed (metal fatigue).
However, the offset weight of the bike causes the wings to rotate. I can't use a roof rack due to the fact that I have a limited range of mobility with one arm.
I've seen hitch racks that range from a $70 Bell to a $500 Draftmaster. What experiences do others have with various hitch racks?